Small high-swirl direct-injection CI engines have fuel conversion efficiencies which are about 10 percent higher than...

Small high-swirl direct-injection CI engines have fuel conversion efficiencies which are about 10 percent higher than values typical of equivalent indirect-injection engines. (ID1 engines are used because they achieve higher bmep.) What combustion-system-related differences contribute to this higher efficiency?

Expert Solution

Comparison between Small high-swirl Direct Injection CI engines and equivalent Indirect Injection engines:

Direct Injection CI engines has typically two design philosophies which help them to have higher fuel conversion efficiency:

1) High-swirl design, which have a deep bowl in the piston, a low number of holes in the injector and moderate injection pressures.

2) Low-swirl or quiescent engines are characterized by having a shallow bowl in the piston, a large number of holes in the injector and higher injection pressures.

Smaller engines tend to be of the high-swirl type, while bigger engines tend to be of the quiescent type.

In direct injection CI engines, there is sufficient time for the fuel to be injected into the cylinder and for it to be distributed and thoroughly mixed with the air charge so that combustion takes place over the most effective crank angle movement just before and after TDC, without having to resort to induction swirl and large amounts of compression squish.

Its because without air swirl in the combustion-chamber there is no high hot gas velocity, which would increase the thermal impingement on the surfaces surrounding the chamber space. Accordingly, there will be more heat available to do useful work so that higher brake mean effective pressures can be obtained where mixing of the fuel and air is achieved purely by the intensity of the spray and its ability to distribute and atomize with the surrounding air.

Whereas in indirect injection engines, when the inlet valve opens and the piston moves away from the cylinder head, air enters the cylinder tangentially so that it rotates in a downward direction about the cylinder axis. Once the piston has reached BDC it reverses its direction and commences to move inwards towards the cylinder head until the inlet valve closes, this then completes the induction period.

As the piston approaches TDC, the air charge is compressed between the cylinder head and the piston crown so that something like 35% to 45% of the air is forced through the five nozzle holes which protrude below the flat cylinder head. Air will then be transferred from the cylinder in to the pre-combustion chamber via the nozzle holes and parallel throat passage where it is exited into a vigorous and highly turbulent mass. Due to this the fuel conversion efficiency of the latter is lesser when compared to the former.

samet mamat answered 2 years ago

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